This invention relates to phase locked loops and more particularly to an automatic sweep and acquisition circuit for such phase lock loops.
In the past, receivers for frequency modulation, for bi-phase phase shift keyed modulation, or for tracking, usually employed a quadrature channel for producing an error signal and an in-phased channel for determining a locked condition or vice versa.
With regard to a Costas phase locked loop, the bandwidth of the loop was 8 kHz or less, while the frequency uncertainty of the translation oscillators in the radio equipment handling the signal was greater than this bandwidth. The major problem encountered was to sense when to turn on the sweep circuit and when to turn it off, since in the case of the Costas loop demodulator, there is no essential difference between the in-phase channel and the quadrature channels and no obvious way to use them to determine in or out of locked condition for the phase locked loop.